Charge coupled device image sensor

ABSTRACT

A CCD image sensor comprising: a semiconductor substrate of a first conductivity type connected to a ground; an impurity region of a second conductivity type formed in the surface of the semiconductor substrate of the first conductivity type, to serve as a blooming prevention layer; an impurity region of the first conductivity type formed in the surface of the semiconductor substrate, so that it encloses the impurity region of the second conductivity type serving as a blooming prevention layer, to serve as a potential barrier layer; an impurity region of the second conductivity type formed in the surface of the semiconductor substrate of the first conductivity type so that it encloses the impurity region of the first conductivity type serving as a potential barrier layer, to serve as a light receiving region; an insulation film which is formed on the surface of the semiconductor substrate of the first conductivity type and has contact holes at both edges of the impurity region of the second conductivity type, serving as a blooming prevention layer; silicide films filled in the contact holes; and a light shield conductor film which is formed on the surface of the remaining insulation film, except for a portion between the silicide films and the surfaces of the silicide films, and is connected to a voltage source.

BACKGROUND OF THE INVENTION

The present invention relates to a charge coupled device image sensorand a method of making the same.

A charge coupled device (CCD) image sensor essentially comprises a lightreceiving region for receiving light and generating a signal charge inresponse to the received light, and a signal charge transfer region fortransferring the generated signal charge in a single direction.

However, the light receiving region generally generates excess signalscharge when too much light is incident on the light receiving region.

The excess signal charge causes a blooming phenomenon to be generated ona screen.

When a signal charge in excess of necessary signal charges is moved fromthe light receiving portion to the signal charge transfer region, thisphenomenon is referred to as an over flow drain (OFD).

Conventionally, the light receiving region of CCD image sensor has astructure for preventing the OFD phenomenon.

The structure of the light receiving region for preventing the OFDphenomenon may generally be classified into a lateral structure and avertical structure.

At present it is preferable to use the vertical structure, as the sizeof the light receiving region is reduced in accordance with the highintegration of CCD image sensor.

Referring to FIG. 1, there is shown a section of a conventional CCDimage sensor which has a light receiving region of the verticalstructure to prevent the OFD phenomenon.

As shown in FIG. 1, the conventional CCD image sensor comprises: asilicon substrate 1 of n-type;

a well 2 of p-type formed in the surface of the silicon substrate 1; afirst insulation film 3 formed on the surface of the well 2;

a signal charge transfer region 4 of n-type formed in the surface of thewell 2; a surface layer 5 of p⁺ -type (⁺ denotes a high concentration)spaced from the signal charge transfer region 4 with a constantdistance, for applying an initial bias;

a light receiving region 6 of n-type formed beneath the surface layer 5of p⁺ -type; a first electrode 7 formed on the surface of the firstinsulation film 3 which is corresponding to the upper part of the signalcharge transfer region 4; a second insulation film 8 formed on thesurface of the first electrode 7;

a second electrode 9 formed on the surface of the second insulation film8 and the surface of the first insulation film 3 which is locatedbetween the signal charge transfer region 4 of n-type and the lightreceiving region 6 of n-type;

a third insulation film 10 formed on the surface of the second electrode9; and

a light shield layer 11 formed on the surface of the first insulationfilm 3 and the third insulation film 10 except for the upper portion ofthe light receiving region 6 of n-type.

Herein, the well 2 of p-type comprises a shallow well 2a and a deep well2b to prevent a smear phenomenon.

The smear phenomenon means that a part of the signal charges generatedfrom a light receiving region corresponding to a pixel are mixed withsignal charges generated from another light receiving regioncorresponding to another pixel, and then moved to the light receivingregion of n-type, corresponding to the other pixel.

Consequently, the smear phenomenon makes it impossible to correctlydisplay an image on a screen and moreover reduces the resolution of thescreen.

The operation of the CCD image sensor shown in FIG. 1 will hereinafterbe described briefly in conjunction with FIG. 1.

First, when light corresponding to an image signal is incident, thelight receiving region 6 generates signal charges corresponding to theimage signal.

At this time, if too much light is incident on the light receivingregion 6, the light receiving region 6 of n-type generates unnecessaryexcess signal charges in excess of the signal charges corresponding tothe image signal.

At this time, if a bias is applied to the silicon substrate 1 of n-typewhich is connected to a voltage source Vd, the saddle point of potentialtaken on the line A--A' is then lowered toward the silicon substrate 1of n-type by a predetermined height.

Therefore, the excess signal charges flow toward the silicon substrate1, thereby enabling the OFD phenomenon and the smear phenomenon to bereduced.

However, the above-conventional technique has the followingdisadvantages.

Firsts the time for making a CCD image sensor is increased and themethod of making a CCD image sensor is complicated, since a p-type wellis essentially formed to prevent the OFD phenomenon.

Second, it is easy for the smear phenomenon to be generated and quantumefficiency is lows since the characteristic of long wavelength is notgood.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of the aboveproblem, and it is an object of the present invention to provide a CCDimage sensor and a method of making the same, in which the process ofmaking it can be more simplified, the characteristic of long wave lengthcan be improved and a smear characteristic can be avoided.

To achieve this object, the present invention uses a substrate of p-typeinstead of a substrate of n-type and also moves the excess signalcharges from the light receiving regions toward the surface side of thesubstrate by applying a bias to the surface of the substrate, instead ofmoving the excess signal charges from the light receiving region towardthe lower portion of the substrate as was explained above for the priorart.

In accordance with one aspect of the present invention, there isprovided a CCD image sensor comprising:

a semiconductor substrate of a first conductivity type connected to aground;

an impurity region of a second conductivity type formed in the surfaceof the semiconductor substrate of the first conductivity type, to serveas a blooming prevention layer;

an impurity region of the first conductivity type formed in the surfaceof the semiconductor substrate, so that it encloses the impurity regionof the second conductivity type serving as a blooming prevention layer,to serve as a potential barrier layer;

an impurity region of the second conductivity type formed in the surfaceof the semiconductor substrate of the first conductivity type, so thatit encloses the impurity region of the first conductivity type servingas a potential barrier layer, to serve as a light receiving region;

an insulation film which is formed on the surface of the semiconductorsubstrate of the first conductivity type and has contact holes at bothedges of the impurity region of the second conductivity type, serving asa blooming prevention layer;

silicide films filled in the contact holes; and

a light shield conductor film which is formed on the surface of theremaining insulation film, except for a portion between the silicidefilms and the surfaces of the silicide films, and is connected to avoltage source.

In accordance with another aspect of the present invention, there isprovided a method of making a CCD image sensor, comprising the steps of:

forming a first insulation film on the whole surface of a semiconductorsubstrate of a first conductivity type;

implanting impurity ions of the first conductivity type having aconcentration higher than that of the semiconductor substrate in thesemiconductor substrate of the first conductivity type, thereby forminga first channel stop region and a second channel stop region of thefirst conductivity type spaced from each other with a constant distancein the surface of the semiconductor substrate;

implanting impurity ions of a second conductivity type in thesemiconductor substrate of the first conductivity type, thereby forminga signal charge transfer region of the second conductivity typecontacted with the first channel stop region, in the semiconductorsubstrate of the first conductivity type;

forming a conductor on the whole surface of the first insulation filmand patterning the conductor, thereby forming a first electrode in theupper portion of the signal charge transfer region;

forming a second insulation film on the whole surfaces of the firstelectrode and the first insulation film, and patterning the secondinsulation film, thereby allowing only a portion of the secondinsulation film which is formed on the surface of the first electrode toremain;

forming a conductor on the whole surface of the remaining secondinsulation film and the first insulation film and patterning theconductor, thereby allowing only a portion of the conductor which isformed on the surface of the remaining second insulation film to remain,as a second electrode;

implanting impurity ions of the second conductivity type in thesemiconductor substrate of the first conductivity type, thereby forminga light receiving region of the second conductivity type in the surfaceof the semiconductor substrate between the second channel stop regionand the lower portion of the second electrode;

implanting impurity ions of the first conductivity type in the lightreceiving region of the second conductivity type, thereby forming apotential barrier layer of the first conductivity type in the lightreceiving region of the second conductivity type;

implanting impurity ions of the second conductivity type having aconcentration higher than that of the light receiving region in thepotential barrier layer of the first conductivity type, thereby forminga blooming prevention layer of the second conductivity type; forming athird insulation film on the whole surfaces of the first insulation filmand the second electrode and patterning the third insulation film,thereby allowing to remain only a portion of the third insulation layerwhich is formed on the surface of the second electrode;

patterning the first insulation film, thereby forming contact holesrespectively at both edge portions of the blooming prevention layer ofthe second conductivity type; filling the contact holes with a metalhaving a high melting point;

annealing the metal filled in the contact holes, thereby converting themetal into a silicide film;

forming a conductor on the whole surfaces of the first insulation film,the third insulation film and the silicide films and patterning theconductor so that only a portion between the silicide films can beremoved, thereby forming a light shield conductor layer; and

forming a protection film on the exposed whole surfaces of the firstinsulation film and the light shield conductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the structure of a conventional CCDimage sensor.

FIG. 2 is a diagram showing a potential profile taken on the line A--A'of FIG. 1.

FIG. 3 is a sectional view showing the structure of a CCD image sensorin accordance with the present invention.

FIG. 4 is a diagram showing a potential profile taken on the line B--B'of FIG. 3.

FIGS. 5a to 5g are sectional views illustrating a method of making a CCDimage sensor in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in detail inconjunction with FIGS. 3 to 5.

Referring to FIG. 3, there is illustrated a sectional view showing thestructure of a CCD image sensor in accordance with the presentinvention.

As shown in FIG. 3, the CCD image sensor comprises: a silicon substrate30 of p-type; an impurity layer 31 of n+-type (herein, ⁺ denotes a highconcentration) formed beneath the surface of the silicon substrate 30,serving as a blooming prevention layer; an impurity layer 32 of p-type(herein, ⁻ denotes a low concentration) formed in the silicon substrate30 of p-type, corresponding to the lower portion of the impurity layer31 of n⁺ -type, so that the impurity layer 32 of p-type encloses theimpurity layer 31 of n⁺ -type, serving as a potential barrier layer;

an impurity layer 33 of n-type formed in the silicon substrate 30 ofp-type, corresponding to the lower portion of the impurity layer 32 ofp-type, so that the impurity layer 33 of n-type encloses the impuritylayer 32 of p-type, serving as a light receiving region; an impuritylayer 34 of n-type formed in the silicon substrate 30 of p-type, so thatone side of the impurity layer 34 is spaced from one side of theimpurity layer 33, serving as a signal charge transfer region;

two impurity regions 35,36 of p⁺ -type formed at the other sides of theimpurity region 33 and the impurity region 34 of n-type, respectively,serving as a channel stop region;

an insulation film 37 which is formed on the surface of the siliconsubstrate 30 of p-type and has contact holes 37a at both edge portionsof the impurity region 31 of n⁺ -type, respectively, serving as ablooming prevention layer;

conductor films 37b filled in the contact holes 37a, respectively; afirst electrode 38 formed on the surface of the insulation film 37corresponding to the upper portion of the impurity layer 34 of n-typeserving as a signal charge transfer region; an insulation film 39 formedon the first electrode 38 so that it encloses the exposed surface of thefirst electrode 38;

a second electrode 40 formed on the insulation film 39, 37 so that itcovers the exposed surface of the insulation film 39 and the surface ofthe insulation film 37 corresponding to between the surface of the lightreceiving region 33 and the surface of the signal charge transferregion;

an insulation film 41, formed so that it covers the exposed surface ofthe second electrode 40; and

an optical shield conductor layer 42, formed on the insulation film 41,the conductor films 37b and the remaining part of the insulation film37, except for a portion between the contact holes 37a, connected to avoltage source Vd and electrically connected to the impurity layer 31 ofn⁺ -type via the conductor films 37b.

Herein, the optical shield conductor layer 42 is made of either a metalor a conductive polysilicon and the conductor films 37b are made of asilicide.

Also, the doping concentrations of p-type and n-type are higher thanthat of p-type and n-type and lower than that of p⁺ -type and n⁺ -type,respectively.

Hereinafter, the operation of the CCD image sensor shown in FIG. 3 willbe described briefly.

As shown in FIG. 3, the silicon substrate 30 is connected to the groundGND and the light shield conductor layer 42 is connected to the voltagesource Vd.

Therefore, upon applying a bias from the voltage source Vd into thelight shield conductor layer 42, the potential is largely lowered at thesurface of the silicon substrate 30 of p-type as shown in FIG. 4, ascompared with other portions.

At this time, the impurity layer 33, serving as a light receivingregion, generates signal charges in response to incident light.

Excess signal charges of the generated signal charges flow toward theground GND via the conductor films 37b and the light shield conductorlayer 42.

As above mentioned, the blooming phenomenon can be restrained, since theexcess signal charges flow toward the surface of the silicon substrate30 of p-type.

As shown in FIG. 4, it is possible to embody a CCD image sensor of highsensitivity since the potential is formed relatively higher than that ofother portions at the bulk portion of p-type silicon substrate 30.

Hereinafter, the method of making the CCD image sensor shown in FIG. 3will be described in detail in conjunction with FIGS. 5a to 5g.

First, a first insulation film 51 is formed on the whole surface of ap-type silicon substrate 50 as shown in FIG. 5a.

Subsequently, p⁺ -type impurity ions are implanted in the p-type siliconsubstrate 50, thereby to form a first p⁺ -type channel stop region 52and a second p⁺ -type channel stop region 52a spaced from each otherwith a constant distance in the surface of p-type silicon substrate 50,as shown in FIG. 5b.

Thereafter, n-type impurity ions are implanted in the p-type siliconsubstrate 50, thereby forming an n-type signal charge transfer region 53in contact with the first channel stop region 52, in the surface of thep-type silicon substrate, corresponding to a portion between the firstp⁺ -type channel stop region 52 and the second p⁺ -type channel stopregion 52a.

Thereafter, a conductive polysilicon is deposited on the surface of thefirst insulation film 51, as a conductor, using a chemical vapordeposition (CVD) method and then the polysilicon is patterned to form afirst electrode 54 on the first insulation film 51 corresponding to theupper portion of the n-type signal charge transfer region 53.

Subsequently, an insulation film is deposited on the exposed wholesurface of the p-type silicon substrate 50 and the first electrode 54using a CVD method and the insulation film is patterned to form a secondinsulation film 55, which covers the surface of the first electrode 54.

Subsequently, a polysilicon is deposited on the exposed whole surface ofthe p-type silicon substrate 50 and the second insulation film 55 usinga CVD method and then the polysilicon is patterned, thereby forming asecond electrode 56 which covers the surface of the second insulationfilm 55.

Thereafter, n-type impurity ions are implanted in the p-type siliconsubstrate 50, forming an n-type light receiving region 57 spaced fromthe n-type signal charge transfer region 53 with a constant distance, inthe surface of the p-type silicon substrate 50 between the second p⁺-type channel stop region 52a and the lower portion of the secondelectrode 56, as shown in FIG. 5c.

As shown in FIG. 5d, subsequently p-type impurity ions are implanted inthe n-type light receiving region 57 to form a p-type barrier region 58in the n-type light receiving region 57 and then n⁺ -type impurity ionsare implanted in the p-type barrier region 58 to form an n⁺ -typeimpurity region 59 in the p-type barrier regions 58.

The n⁺ -type impurity region 59 makes it possible to move the excesssignal charges toward the ground GND, thereby enabling the bloomingphenomenon to be prevented.

Thereafter, an insulator is deposited on the exposed whole surface ofthe second electrode 56 and the first insulation film 51 using a CVDmethod and then the insulator is patterned to form a third insulationfilm 60 which covers the surface of the second electrode 56, as shown inFIG. 5e.

The first insulation film 51 is then patterned to form contact holes51a, 51b at both edges of the n⁺ -type impurity region 59, respectively.

Subsequently, a metal having a high melting point is deposited in thecontact holes 51a, 51b so that the metal fills the contact holes 51a,51b.

The metal having a high melting point is then subjected to an annealingprocess to convert the metal into silicide films 61, 61a, respectively.

Subsequently, a conductor 62 is deposited on the exposed whole surfacesof the first insulation film 51, the third insulation film 60 and thesilicide films 61,61a by using a CVD method.

As shown in FIG. 5f, the conductor 62 is then patterned, thereby toremove a portion corresponding to the upper portion of the lightreceiving region 57 and as a result, forms a light shield conductor film63.

Subsequently, a protection film 64 is formed on the exposed surfaces ofthe first insulation form 51, the silicide film 61, 61a and the thirdinsulation film 60 using a CVD method, as shown in FIG. 5g.

Herein, the conductor 62 may be made of a metal or a conductivepolysilicon.

An insulator may be used as a material for the protection film 64.

According to the present invention, as above mentioned, the followingeffects can be obtained.

First, the process of making a CCD image sensor can be more simplified,as compared with that of a conventional CCD image sensor.

Second, defects of the p-type silicon substrate can be prevented, sinceit is not required to anneal the silicon substrate at a high temperaturefor a long time.

Third, the light sensitivity can be improved, since a higher relativepotential is formed at the bulk side of the p-type silicon substrate, ascompared with other portions.

Fourth, it is possible to reduce the smear phenomenon due to opticalcross-talk, since the light shield conductor film 63 is directlycontacted with silicide films 61, 61a.

What is claimed is:
 1. A CCD image sensor comprising:a semiconductorsubstrate of a first conductivity type connected to a ground; animpurity region of a second conductivity type formed in a surface of thesemiconductor substrate of the first conductivity type, to serve as ablooming prevention layer; an impurity region of the first conductivitytype formed in the surface of the semiconductor substrate, so that itencloses the impurity region of the second conductivity type serving asa blooming prevention layer, to serve as a potential barrier layer; animpurity region of the second conductivity type formed in the surface ofthe semiconductor substrate of the first conductivity type, so that itencloses the impurity region of the first conductivity type serving as apotential barrier layer, to serve as a light receiving region; aninsulation film which is formed on the surface of the semiconductorsubstrate of the first conductivity type and has contact holes at bothedges of the impurity region of the second conductivity type, serving asa blooming prevention layer; silicide films filled in the contact holes;and a light shield conductor film which is formed on a surface of theinsulation film, except for a portion between the silicide films and thesurfaces of the silicide films, and is connected to a voltage source. 2.A CCD image sensor as set forth in claim 1, wherein the firstconductivity type is p conductivity type and the second conductivitytype is n conductivity type.
 3. A CCD image sensor as set forth in claim1, wherein the impurity region of the second conductivity type servingas a blooming prevention layer has a concentration higher than that ofthe impurity region of the second conductivity type serving as a lightreceiving region.
 4. A CCD image sensor, as set forth in claim 1,wherein the light shield conductor film is made of either a metal or aconductive polysilicon.
 5. A CCD image sensor, as set forth in claim 1,wherein the semiconductor substrate of the first conductivity type ismade of silicon.